<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.4.1/jquery.js"></script>
</head>
<body>
    <div id="test" style="display:block">hello world</div>
    <button id="btn">切换</button>
    <script>
        var btn=document.getElementById("btn")
        var test=document.getElementById("test")
  /*       btn.onclick=function(){
            var value=test.style.display;
            // console.log(value)
            if(value=="block"){
                test.style.display="none"
            }else{
                test.style.display="block"
            }
        }
 */
        //用jquery写
        $("#btn").click(function(){
            /* is(":visible")判断一个元素是否显示 */
            var isShow=$("#test").is(":visible")
            if(isShow){
                $("#test").hide(300)
            }else{
                $("#test").show(300)
            }
        })
    </script>
</body>
</html>